I was driving home from the city a while back, thinking about (of all things), gravity, dark energy and the expansion of the universe. I know, rather odd things to be thinking about on an hour and a half long drive, but hey, that's me.
The theory I came up with is this, of which I have no clue if it's possible, probable, or I'm just totally out to lunch.
Scientists have been searching for the cause of the increasing speed of the expansion of the universe for some time now, dubbing the force responsible for it "dark energy".
Dark Energy Wikipedia Link My thought was...what if this expansion isn't caused by some unknown force, but instead is simply another aspect of an already familiar, but far from understood force. Gravity.
Everyone knows that, on the scale of us, our planet, our solar system, even super clusters of galaxies, gravity tends to reign supreme, holding everything in a relatively cohesive unit. However, it weakens as the square of the distance between two objects. So over massive distances, gravity from an object is essentially nil on another distant object. Unless I'm terribly wrong, gravitational force is currently thought to approach zero, but never actually reach zero (to become infinitely close, but never reach).
What if, instead of simply becoming incredibly weak over massive (billions of light-years) distance, gravity essentially reversed itself. It's look something like this...please excuse the horrible little graph, I whipped it up in about 3 minutes with Paint
So instead of a mysterious force permeating space, causing an expansion of increasing speed, gravity, once the universe expanded sufficiently from the Big Bang, would expand due to, of all things, the gravity that also holds it together.
Now, feel free to blow holes in my wee theory, in fact, I hope you do. I just thought it was an interesting idea, that's more than likely impossible. Though, we really don't have any idea of how gravity actually 'works', so...who knows!